Water Treatment
Cycles of Concentration and Blowdown, Explained Properly
Evaporation leaves the dissolved solids behind. How far you let them concentrate decides your water bill, your scaling risk and your discharge volume.
Why anything concentrates at all
A cooling tower works by evaporating water. What evaporates is pure: the dissolved salts, hardness and silica stay behind in the circulating water. Make up water is added to replace what left, and it brings its own dissolved solids with it.
The result is that the circulating water becomes steadily more concentrated than the make up. Cycles of concentration is simply the ratio between the two. At five cycles, the circulating water carries five times the dissolved solids of the incoming supply.
In practice cycles are measured by comparing something conservative in both streams, usually chlorides or conductivity, because those pass through without precipitating.
The three water streams
Only three things happen to water in an open recirculating tower, and they balance.
- Evaporation: roughly 1.8 litres per hour for every ton of refrigeration, or about 1% of circulation for every 5.5 degrees of range.
- Drift: water carried out as droplets in the leaving air. Modern eliminators hold this well below 0.05% of circulation, so it is usually small enough to ignore in a first calculation.
- Blowdown: water deliberately removed to stop the concentration climbing further.
The calculation
Blowdown equals evaporation divided by cycles minus one. Make up equals evaporation plus blowdown. Those two relationships are all you need.
Take a 375 TR tower. Evaporation is roughly 375 times 1.8, which is 675 litres per hour. Running at 3 cycles, blowdown is 675 divided by 2, which is 338 litres per hour, and make up is 1013 litres per hour.
Run the same tower at 6 cycles and blowdown falls to 675 divided by 5, which is 135 litres per hour. Make up drops to 810 litres per hour. That is a 20% cut in water consumption from a single operating decision, with no change to the equipment.
Why sites run at low cycles anyway
The saving above is real and widely known, so it is worth asking why so many plants still run at two or three cycles.
The honest answer is that low cycles are a way of managing scale by dilution. If nothing is controlling the calcium carbonate, the only lever is to keep concentration low by dumping water. It works, and it is expensive.
Raising cycles without addressing the scaling risk simply moves the problem from the water bill to the condenser tubes. That is why the cycles decision and the treatment decision have to be made together rather than separately.
What actually limits cycles
Cycles are limited by whichever dissolved species reaches its solubility limit first in your water. For most Indian supplies that is calcium carbonate, driven by calcium hardness and alkalinity. On some bore well supplies it is silica, which is far less forgiving because it forms a glassy deposit that acid will not remove.
A water analysis tells you which constraint you are actually against. Without one, any statement about achievable cycles is guesswork.
Where calcium carbonate is the limit, non-chemical conditioning raises the ceiling substantially, because the hardness precipitates as a suspension that leaves with blowdown rather than bonding to the tubes. Where silica is the limit, conditioning does not help and the ceiling stands.
A sensible operating routine
Measure conductivity in the make up and in the basin, and divide one by the other to get your actual cycles. Do it monthly and write it down, because the number drifts as supply water changes through the year.
Check that the blowdown line is genuinely flowing. A silted or closed bleed is one of the most common faults we find, and it drives concentration up quietly until scale appears.
Set a conductivity target based on your water analysis rather than a number copied from another plant, and control blowdown against it.
